The development of microwave cooking has had an enormous impact on both commercial and industrial food preparation. The high speed with which cooking occurs and the broad array of materials suitable for use in microwave ovens has created an ever-increasing demand for economical, simple, disposable containers which, when used in a microwave oven, duplicate as closely as possible the cooking results of a conventional oven. Such containers must be capable of serving as a shipping carton which is suitable for display in a supermarket and must be usable as a heating dish by the ultimate consumer. At the same time, it is essential that the container be sufficiently economical to allow it to be disposed after a single use. Paperboard cartons satisfy many of these characteristics but must be designed in accordance with requirements which are dependent on the type and quantity of food to be placed in the container. In the case of pizza, a satisfactorily cooked product must have a crisp crust while avoiding an overcooked, scorched, burned or charred effect. Moreover, the topping of the pizza must be heated properly throughout and must not be dried or burned.
To achieve these goals, a microwave cooking container for pizza needs to allow ventilation of the product as well as air circulation to prevent trapping of excess moisture which might produce a leathery or soggy consistency. An early attempt to produce a paper carton for pizza is illustrated in the patent to Tolaas (U.S. Pat. No. 3,876,131). The carton of this patent is provided with a series of apertures for ventilation which are normally closed by a removable film overlay. While the Tolaas carton functions desirably for its intended purpose, the need to secure the overlay film thereto is undesirable from a manufacturing standpoint. Not only does its application require a separate operation, but it also introduces into the manufacturing procedure the necessity for handling a material unlike that from which the carton body is fabricated, and the use of a film may require the provision of special means to enable facile removal.
The Tolaas patent further teaches the desirability of supporting the pizza above the surface of the microwave oven to achieve a more even cooking of the pizza crust. While the Tolaas carton has structure providing this function, the elevating elements thereof extend beneath the body of the carton in its erected configuration, thus increasing the volume occupied by the carton (such as during transport) and subjecting those elements to possible damage and distortion.
A substantial improvement over the Tolaas carton design and a remedy for many of its deficiencies is disclosed in the patent to Kuchenbecker (U.S. Pat. No. 4,096,948) in which is disclosed a carton having ventilation openings normally closed by a removable panel section. This section is integral with the carton blank eliminating the overlay film problem of Tolaas. Furthermore, by removing the section, two tabs were exposed which could be folded along score lines so as to provide support legs for the carton. This design feature reduced the overall volume of the carton and simplified its shipping requirements. Despite these important steps forward in package integrity and design simplicity, an ideal microwave carton meeting all of the above-described characteristics had not yet been disclosed.
Still another package for producing acceptable pizza from a microwave oven has been disclosed by Turpin, as shown in U.S. Pat. No. 4,190,757. This patent teaches that by placing pizza on a microwave energy absorber such as an interactive layer which converts microwave into heat and inserting holes in the top of a carton having some microwave shielding, a crisp pizza crust could be obtained. However, the package disclosed in Turpin's patent is complicated and expensive to produce. The use of any type of microwave reflective or absorptive material in a paperboard carton always presents the possibility of charring or even burning of the food and/or the paperboard material of which the carton is made. Attempts to avoid the danger of charring of the paperboard have normally resulted in the use of an entirely separate element arranged to be placed between the food and the food support or in a complicated outline for the microwave interactive layer which is different from the outline of the paperboard blank on which the interactive layer is supported such as illustrated in the patent to LePothier (U.S. Pat. No. 3,865,301). These prior designs for the microwave interactive layer can add significantly to the cost of the final carton. Furthermore, it has been discovered that pizza can stick to a microwave interactive layer, making removal difficult. Similar expense and complexity problems have been encountered in producing other types of cartons such as disclosed in the patent to Winters (U.S. Pat. No. 4,283,427) although this patent does suggest a solution to the sticking problem by teaching the application of a substantially grease-resistant material to the top of an insert pouch on which pizza would be cooked. Further disclosure concerning stick-resistant coatings is discussed in the patent to Webinger (U.S. Pat. No. 4,279,374) but this reference in no way suggests solutions to the many other problems associated with producing an ideal microwave pizza carton.
Thus, it has remained an elusive goal in the microwave container art to produce a "cook-in" container for pizza which is inexpensive and simple to manufacture yet still results in a high quality crisp crust pizza that is easily accessible and removable.